Various flat panel displays such as a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro luminescence (EL) display have recently emerged as candidate displays capable of replacing a conventional cathode ray tube.
Among devices used for these flat panel displays, an organic light emitting device is in the spotlight as a device suitable for a next-generation flat panel display because it has superior characteristics including a low driving voltage of 3 to 10V, a wide viewing angle, high-speed response, a high contrast ratio and so forth. This organic light emitting device is a device in which, if electric charges are injected into an organic emission layer formed between an electron injection electrode (negative electrode) and a hole injection electrode (positive electrode), electrons and holes meet each other to generate excitons, and light emission occurs when the excitons fall again to the ground state.
Such an organic light emitting device generally has a structure in which a cathode electrode (negative electrode) and an anode electrode (positive electrode) are disposed opposite to each other on a transparent substrate such as a glass substrate, and an organic emission layer is formed therebetween. An electron or hole transportation layer and an electron or hole injection layer may be additionally interposed between the electrodes and the emission layer. According to the prior art, a transparent electrical conductive electrode having a high work function is used as the positive electrode in order to satisfy the positive electrode's functions of supplying holes and transmitting emitted light to the outside, and such an electrode may be formed by sputtering an ITO (Indium-Tin Oxide) film, an IZO (Indium-Zinc Oxide) film or the like. The negative electrode functions to supply electrons, and a metal thin film having low work function may be formed as the negative electrode by means of a thermal evaporation process, etc.
A display using an organic light emitting device may be classified into a passive matrix (PM) type and an active matrix (AM) type according to a driving method thereof. In the passive matrix type, the organic light emitting device is placed in a portion where bus lines of positive and negative electrodes intersect each other, and is driven in a line-by-line scanning manner. In the active matrix type, ON/OFF adjustment is performed on a pixel-by-pixel basis by using at least one thin film transistor (TFT), and storage capacitance is used for storing information. Further, the active matrix type has advantages in that a process of forming a unit pixel is simpler than in the passive matrix type, and a high definition panel can be manufactured.
A method of manufacturing an organic light emitting device includes a process of forming a bottom electrode on a glass substrate. The bottom electrode is formed directly on the glass substrate in the case of the passive matrix (PM) type, and is formed after fabricating a thin film transistor on the glass substrate in the case of the active matrix (AM) type.
The organic light emitting device may also be classified into a bottom emission type and a top emission type according to a light emission direction thereof. The bottom emission type has a structure in which light is emitted downward, and the top emission type has a structure in which light is emitted upward. In the bottom emission structure, when a TFT is formed below a substrate formed with an emission layer, a small light emitting area cannot be avoided owing to the TFT. On this account, an organic light emitting device having the top emission structure is better in light emission efficiency than an organic light emitting device having the bottom emission structure, and thus is actually more suitable for a display.
However, in view of manufacturing processes, there is a problem in that, when an ITO layer, that is, an anode, is formed by sputtering after an organic material layer is formed, the organic material is damaged by the sputtering, which results in deterioration in the performance of an organic light emitting device. Therefore, in a common manufacturing method of an active matrix type organic light emitting device, it is generally practiced that the ITO layer, that is, an anode, is formed as a bottom electrode. Nevertheless, some investigators have created an organic material capable of enduring the damage caused by sputtering, and have succeeded in developing a so-called inverse structure in which a cathode is formed as a bottom electrode and an anode formed by sputtering is used as a top electrode, so that it has become possible to easily manufacture a top emission structure having good light emission efficiency.